Issue Converting Matlab Code to Python when trying to sum array

Question

I am converting a Matlab code to Python but facing an issue in below lines:

Code:

Matlab:

P_asef_t = sum(P_asef);
P_aseb_t = sum(P_aseb);

Python:

import numpy as np
import scipy

P_asef_t = np.sum(P_asef)
P_aseb_t = np.sum(P_aseb)

Matlab

whos P_asef
Variables in the current scope:

   Attr Name        Size                     Bytes  Class
   ==== ====        ====                     =====  ===== 
        P_asef     51x1200                  489600  double

Total is 61200 elements using 489600 bytes

And in Python:

(Pdb) P_asef.shape, P_asef.size
((51, 1200), 61200)

But variable P_asef_t comparatively is coming completely incorrect:

MATLAB:

debug> whos P_asef_t
Variables in the current scope:

   Attr Name          Size                     Bytes  Class
   ==== ====          ====                     =====  ===== 
        P_asef_t      1x1200                    9600  double

Total is 1200 elements using 9600 bytes

debug>  P_asef_t
P_asef_t =

 Columns 1 through 6:

   1.2208e-11   9.9358e-03   9.8720e-03   9.8087e-03   9.7457e-03   9.6831e-03

 Columns 7 through 12:

   9.6210e-03   9.5592e-03   9.4978e-03   9.4368e-03   9.3762e-03   9.3160e-03

 Columns 13 through 18:

Python:

(Pdb) P_asef_t
1.3898510532602344
(Pdb) P_asef_t.shape, P_asef_t.size
((), 1)
(Pdb)

How can I fix this issue?

spfrnd · Accepted Answer · 2015-02-02 07:43:18Z

2

Numpy's sum function takes an additional argument axis, that defines over which axis of the array is summed. The problem here is that is defaults to summing over all axes.

In the case of a matrix such as P_asef you have two axes. The 0th axis are the columns and the 1st are the rows. If you only want to sum along columns, you need to tell sum to only sum over axis=0.

import numpy as np

mat = np.ones((51,1200))
mat1 = np.sum(mat,axis=0)
mat2 = np.sum(mat,axis=1)
mat3 = np.sum(mat)

answered Feb 2, 2015 at 7:43

spfrnd

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